Selection Principles, Dosage and Usage of Powder Surface Modifiers

The surface modification of powders is largely achieved through the action of surface modifiers on the powder surface. Therefore, the formulation of surface modifiers (variety, dosage and usage) has an important influence on the modification effect of the powder surface and the application performance of the modified product. The formulation of surface modifiers is highly targeted, mainly including the selection of varieties, determination of dosage and usage.

1. Screening of surface modifiers

(1) Properties of powder raw materials The properties of powder raw materials are mainly acidity, alkalinity, surface structure and functional groups, adsorption and chemical reaction characteristics, etc. Surface modifiers that can react chemically or chemically adsorb with the surface of powder particles should be selected as much as possible, because physical adsorption is easy to desorb under strong stirring or extrusion during subsequent application.

For example, the surfaces of acidic silicate minerals such as quartz, feldspar, mica, and kaolin can bond with silane coupling agents to form a relatively strong chemical adsorption; while titanate and aluminate coupling agents can chemically adsorb with carbonate alkaline minerals under certain conditions and to a certain extent.

(2) Product use The use of the product is the most important consideration in selecting a surface modifier. The compatibility and compatibility with the components of the application system must also be considered. For example:

To enhance the compatibility of inorganic powders with organic polymers and their dispersibility in organic matter to improve the mechanical strength and comprehensive performance of the material, organic surface modification can be selected;

To obtain new mineral intercalation compounds, such as clay or graphite intercalation compounds, intercalation modification can be selected;

To replace white carbon black and supplement the deficiencies of white carbon black in certain properties, surface coating with silica can be selected;

To replace titanium dioxide or reduce the amount of titanium dioxide, surface coating with titanium dioxide can be selected;

To improve certain special properties of rubber products, surface coating with metal particles can be selected;

To improve the optical benefits and visual effects of the product, surface coating with metal oxides such as titanium oxide, chromium oxide, and iron oxide can be selected.

(3) Modification process The modification process is also one of the important considerations in selecting a surface modifier, such as temperature, pressure, and environmental factors. All organic surface modifiers will decompose at a certain temperature. For example, the boiling point of silane coupling agent varies between 100 and 310 °C depending on the type. Therefore, the decomposition temperature or boiling point of the selected surface modifier should preferably be higher than the processing temperature during application.

For wet processes, the water solubility of the surface modifier should be considered, because only when it is soluble in water can it fully contact and react with the powder particles in a wet environment. For example, when calcium carbonate powder is surface modified by wet method, if stearic acid is directly added, it is not only difficult to achieve the expected surface modification effect (mainly physical adsorption), but also the utilization rate is low, the surface modifier is seriously lost after filtration, and the organic matter emission in the filtrate exceeds the standard. Therefore, for surface modifiers that cannot be directly soluble in water but must be used in a wet environment, they must be saponified, ammonized or emulsified in advance so that they can be dissolved and dispersed in aqueous solution.

(4) Price and environmental factors Under the premise of meeting the application performance requirements or optimizing the application performance, try to choose a cheaper surface modifier to reduce the cost of surface modification. At the same time, pay attention to choosing a surface modifier that does not cause pollution to the environment.

2. Dosage of surface modifier

Theoretically, the dosage required to achieve monolayer adsorption on the particle surface is the optimal dosage, which is related to the specific surface area of the powder raw material and the cross-sectional area of the surface modifier molecule, but this dosage is not necessarily the dosage of the surface modifier when 100% coverage is achieved. For inorganic surface coating modification, different coating rates and coating layer thicknesses may show different characteristics, such as color, gloss, etc. Therefore, the actual optimal dosage should be determined through modification tests and application performance tests. This is because the dosage of the surface modifier is not only related to the dispersion of the surface modifier during surface modification and the uniformity of coating, but also to the specific requirements of the application system for the surface properties and technical indicators of the powder raw materials.

For wet modification, the actual coating amount of the surface modifier on the powder surface is not necessarily equal to the dosage of the surface modifier, because there is always a part of the surface modifier that fails to interact with the powder particles and is lost during filtration. Therefore, the actual dosage should be greater than the dosage required to achieve monolayer adsorption.

3. How to use surface modifiers

The method of using surface modifiers is one of the important components of the surface modifier formula and has an important impact on the surface modification effect of powders. A good method of use can improve the dispersion degree of surface modifiers and the surface modification effect of powders. On the contrary, improper method of use may increase the amount of surface modifiers used and the modification effect cannot achieve the expected purpose.

The method of using surface modifiers includes preparation, dispersion and addition methods as well as the order of adding when using more than two surface modifiers.

(1) The preparation method of surface modifiers depends on the type of surface modifiers, modification process and modification equipment. Different surface modifiers require different preparation methods. For example, for silane coupling agents, it is silanol that bonds with the surface of powders. Therefore, to achieve a good modification effect (chemical adsorption), it is best to hydrolyze before adding. For other organic surface modifiers that need to be diluted and dissolved before use, such as titanate, aluminate, stearic acid, etc., corresponding organic solvents such as anhydrous ethanol, toluene, ether, acetone, etc. should be used for dilution and dissolution. For organic surface modifiers such as stearic acid, titanate, aluminate, etc. that are not directly soluble in water used in the wet modification process, they should be saponified, ammonized or emulsified in advance to become products that can be dissolved in water.

(2) Addition method The best way to add surface modifiers is to make the surface modifiers and powders in uniform and sufficient contact to achieve a high degree of dispersion of the surface modifiers and uniform coating of the surface modifiers on the particle surface. Therefore, it is best to use a continuous spray or drip (addition) method linked to the powder feeding speed. Of course, only a continuous powder surface modifier can achieve continuous addition of surface modifiers.

The preparation method of inorganic surface modifiers is relatively special, and it is necessary to consider a variety of factors such as solution pH, concentration, temperature, and additives. For example, when titanium dioxide is coated on the surface of muscovite, titanyl sulfate or titanium tetrachloride must be hydrolyzed in advance.

(3) Order of adding agents When more than two surface modifiers are used to treat the powder, the order of adding agents also has a certain influence on the final surface modification effect. When determining the order of adding surface modifiers, it is first necessary to analyze the role of each of the two surface modifiers and the mode of action on the powder surface (whether physical adsorption is the main method or chemical adsorption is the main method). Generally speaking, the surface modifier that plays the main role and is mainly chemically adsorbed is added first, and the surface modifier that plays the secondary role and is mainly physical adsorption is added later.

For example, when a coupling agent and stearic acid are mixed, generally speaking, the coupling agent should be added first and the stearic acid should be added later, because the main purpose of adding stearic acid is to enhance the hydrophobicity and lipophilicity of the powder and reduce the amount of coupling agent used and the cost of the modification operation.